Spin,
Your explanation is also correct, it just depends on if you are looking at the blade or the whole helo. In effect, when the blades lead/lag in unison they move their effective CG away from the mast, they are going as you describe. But this blade motion is because they are each individually not properly damped as they lead/lag, so the real source is that blade motion that resonates with the fuselage rocking motion (usually in roll, because the roll inertia is much, much lower than the pitch inertia.) In fact, most causes of ground resonance are due to one bad damper and one bad blade.